V-belt or toroidal continuously variable transmissions are known in the prior art as continuously variable transmissions used in vehicles. Variable control valves are driven by actuators such as step motors and the variable ratio is continuously varied on the basis of oil pressure.
It is not possible to correctly perform feed back control in this kind of variable control device when the oil temperature of the working oil is extremely low because the viscosity of the working oil increases and the response characteristics of the control valves are greatly reduced or when the actual pulley ratio i.e. the variable ratio, can not be detected due to extremely low speed running. In this event, open loop control exhibits greater accuracy. Hence JPA-9-199524 filed by the present applicant shows the use of open loop control at times of low speed running or low temperatures, such use reverting to feed back control when conditions return to normal.
JP-A-9-329229, JP-A-9-89494 and JP-A-9-89496, filed by the present applicant, are examples of devices provided with an external disturbance compensator to improve normal response characteristics and control external disturbances to the variable mechanism, the influence of age softening and lags in the response time of the step motor.
However convention variable control devices for a continuously variable transmissions initialize the external disturbance compensator of the control device and the like to the actual pulley ratio (Aip) which equals the actual variable ratio, when the transition is made from open loop control to feed back control. In other words, when feed back control is commenced, adjustment is made so that the deviation of the command pulley ratio (ip.sub.R) from the actual pulley ratio (Aip) is equal to 0.
However at this time, it is not certain that the command pulley ratio (ip.sub.R), that is to say the feed back command value, and the actual pulley ratio (Aip) will agree. For example, if the variable ratio is undergoing extreme variation, the deviation easily increases. When there is a large deviation, the command pulley ratio (ip.sub.R) will rapidly change on initialization from the value during open loop control. Furthermore if the command value during open loop control is in agreement with the value during feed back control, no rapid change will occur in the command value when switching.
When the command pulley ratio (ip.sub.R) rapidly changes, the actuator controlling the variable ratio operates rapidly and there is the possibility that a shock will be generated due to the large variation in th e variable ratio.